Avantages
Opportunity to work in various countries on interesting projects. Also, a good opportunity to work with people of different backgrounds, nationalities, beliefs and etc.
Inconvénients
Secondment system really sucks. If you are from a poor country then be ready to live on Board and Lodging Allowances (BLA) only. However, international contracted mission members get UN net salary commensurate with their grade and BLA. Poor management in some departments/units. Objectives are sometimes set unclear and that leads to misunderstanding or event conflicts with colleagues/supervisors. I notices that some international contracted positions (but not only) are occupied by insiders. OSCE makes a public announcement on a vacancy, hundreds of people apply and... an inside candidate is selected. Someone can say that is a coincidence . However, if you make some analysis, you can find that 60 to90% (dependent on certain executive structure) of international contracted posts are occupied by insiders.